The present invention relates to the control of pixel colors in a video display system, and more particularly, the arrangement of electronic elements to implement transparency and drawing modes in contemporary graphic controllers of work stations or advanced personal computer systems.
Graphic control chips suitable to generate patterns for the color video displays used with computer systems exist in various forms. Specific examples which form the background of the present invention may be found in the NCR 7300 color graphics controller chip and its companion NCR 7301 memory interface controller chip. Information regarding the architecture and functional attributes of such controllers is broadly disseminated to those who routinely practice in the art using data sheets and application publications which promote the devices. Furthermore, with the diversity of graphic controller products on the market, numerous structural and functional aspects of such graphic controllers are common to broad cross-sections of the product lines.
The function of such graphic controllers is to translate relatively high level graphic commands from computer microprocessors into graphic chip machine level routines which control the colors of the individuals pixels appearing on the video display. The colors of the pixels on the video display are commonly defined by corresponding binary data stored in a frame buffer memory which is raster scanned in synchronism with the video display. The creation and alteration of the binary data in the frame buffer between raster scanning operations are the activities of the color graphics control system.
The invention at hand is directed to circuitry which performs two functions heretofore requiring elaborate software manipulations. One involves the provision of a transparency mode. The other is a logical drawing mode. The outcome of each mode is reflected in the binary data held by the frame buffer and visually discernible on the color video display.